designating or of the third and last geologic epoch of the Paleogene, characterized by cold climate, the formation of vast grasslands, and the further development of mammals
oligo- + -cene
See Oligocene in American Heritage Dictionary 4
Of or belonging to the geologic time, rock series, or sedimentary deposits of the third epoch of the Tertiary Period, characterized by futher development of modern mammalian fauna, including the rise of the true carnivores and their gradual replacement of the creodonts. See Table at geologic time.